Memory devices using non-volatile memories such as USB memories, memory cards and solid state drives are widely used. Since these memory devices store user data, there has been a demand for the memory devices to provide reliability for storing data for a long time.
There is a tendency that user data, such as images, music and video, which is to be stored in memory devices, increases. Thus, it has been requested that memory devices have large capacities. Processes for manufacturing non-volatile memories have been aggressively shortened.
As conventional techniques related to the present invention, there are techniques that are disclosed in the following patent documents, for example. Accordingly, Japanese Laid-open Patent Publication No. 2004-014043 discusses a technique that a nonvolatile semiconductor executes write of data to the cell transistors at a voltage lower than an ordinary write voltage so as to change a threshold value of the cell transistors in analog fashion. Japanese Patent No. 2509297 discusses a technique that a nonvolatile memory device detects data errors arising from loss of charge in the floating gates of memory cells and corrects by applying two different sense voltages to the memory cells and comparing the outputs. Japanese Patent Laid-open Publication No. 2006-004534 discusses a technique that the ferroelectric storage device generates the reference potential by using a dummy cell.